Phytoremediation potential of alimurgic plants in metal-contaminated environments

As soil metal contamination is rapidly increasing worldwide, mainly because of human activities, phytomanagement of polluted land is becoming an environmentally sustainable and cost-effective option that can also produce biomass and energy as secondary utilities. Some Asteraceae and Polygonaceae species growing spontaneously as invasive weeds on natural and farmed land, which are considered medical or edible alimurgic species, have been identified to accumulate high above-ground levels of toxic elements, thanks to efficient root-to-shoot translocation. Most of them combine high adaptability to marginal soils with good shoot biomass, and many are metal indicator or hyperaccumulator species. In this study were investigated the shoot and root growth potential, metal uptake and translocation at the flowering stage of wild chicory (Cichorium intybus L.), common sowthistle (Sonchus oleraceus L.), salsify (Tragopogon porrifolius L.), common dandelion (Taraxacum officinale Web.) and garden sorrel (Rumex acetosa L.) in artificially highly Cd-Co-Cu-Pb-Zn-contaminated soil

Phytoremediation opportunities with alimurgic species in metal-contaminated environments

Alimurgic species are edible wild plants growing spontaneously as invasive weeds in natural grassland and farmed fields. Growing interest in biodiversity conservation projects suggests deeper study of the multifunctional roles they can play in metal uptake for phytoremediation and their food safety when cultivated in polluted land. In this study, the responses of the tap-rooted perennial species Cichorium intybus L., Sonchus oleracerus L., Taraxacum officinaleWeb., Tragopogon porrifolius L. and Rumex acetosa L. were studied in artificially-highly Cd-Co-Cu-Pb-Zn-contaminated soil in a pot-scale trial, and those of T. officinale and R. acetosa in critical open environments (i.e., landfill, ditch sediments, and sides of highly-trafficked roads). Germination was not inhibited, and all species showed appreciable growth, despite considerable increases in tissue metal rates. Substantial growth impairments were observed in C. intybus, T. officinale and T. porrifolius; R. acetosa and S. oleracerus were only marginally affected. Zn was generally well translocated and reached a high leaf concentration, especially in T. officinale (~600 mg/kg dry weight, DW), a result which can be exploited for phytoremediation purposes. The elevated Cd translocation also suggested applications to phytoextraction, particularly with C. intybus, in which leaf Cd reached ~16 mg/kg DW. The generally high root retention of Pb and Cu may allow their phytostabilisation in the medium-term in no-tillage systems, together with significant reductions in metal leaching compared with bare soil. In open systems, critical soil Pb and Zn were associated with heavily trafficked roadsides, although this was only seldom reflected in shoot metal accumulation. It is concluded that a community of alimurgic species can serve to establish an efficient, long-lasting vegetation cover applied for phytoremediation and reduction of soil metal movements in degraded environments. However, their food use is not recommended, since leaf Cd and Pb may exceed EU safety thresholds

Assessing biochar ecotoxicology for soil amendment by root phytotoxicity bioassays

Soil amendment with biochar has been proposed as effective in improving agricultural land fertility and carbon sequestration, although the characterisation and certification of biochar quality are still crucial for widespread acceptance for agronomic purposes. We describe here the effects of four biochars (conifer and poplar wood, grape marc, wheat straw) at increasing application rates (0.5, 1, 2, 5, 10, 20, 50 % w/w) on both germination and root elongation of Cucumis sativus L., Lepidium sativum L. and Sorghum saccharatum Moench. The tested biochars varied in chemical properties, depending on the type and quality of the initial feedstock batch, polycyclic aromatic hydrocarbons (PAHs) being high in conifer and wheat straw, Cd in poplar and Cu in grape marc. We demonstrate that electrical conductivity and Cu negatively affected both germination and root elongation at 655 % rate biochar, together with Zn at 6510 % and elevated pH at 6520 %. In all species, germination was less sensitive than root elongation, strongly decreasing at very high rates of chars from grape marc (>10 %) and wheat straw (>50 %), whereas root length was already affected at 0.5 % of conifer and poplar in cucumber and sorghum, with marked impairment in all chars at >5 %. As a general interpretation, we propose here logarithmic model for robust root phytotoxicity in sorghum, based on biochar Zn content, which explains 66 % of variability over the whole dosage range tested.We conclude that metal contamination is a crucial quality parameter for biochar safety, and that root elongation represents a stable test for assessing phytotoxicity at recommended in-field amendment rates (<1\u20132 %)

Improving phytoremediation efficiency in metal-polluted wastes

Human activities are responsible for the release of pollutants into the environment, and soil contamination by metals and metalloids is of primary concern. Currently, the laws covering environmental management and protection of several countries are focusing on the less expensive and more environmentally friendly biological methods of restoration, compared with the more invasive physico-chemical ones. The present study regards the first attempt in Italy to apply in situ phytoremediation of a metal-polluted, markedly degraded area located at Torviscosa (Udine, NE Italy) and included in the national priority list of sites for restoration. Within the site, pyrite cinders, derived from mineral roasting for sulphur extraction, had been severely contaminated by various metals such as arsenic, cadmium, copper, lead and zinc. The wastes were discharged in the Thirties and covered with an unpolluted 0.15-m layer of gravelly soil. Because of the particular substrate composition, poor physical structure, nutrient deficiency, and extreme hydrological conditions, the site was colonised by a rare and inadequate vegetation cover. Application of phytoremediation with biomass species required particular care in soil management and adjustment of the cultivation method. In this regard, ploughing was compared with subsoiling, in order to dilute the cinders with the unpolluted top layer and to maintain a shallow favourable layer for plant establishment, respectively. In situ, trials were carried out to compare woody species (Populus alba L., P. nigra L., P. tremula L., Salix alba L.) and crops (Helianthus annuus L., Lolium multiflorum Lam., Medicago sativa L., Raphanus sativus L. var. oleiformis Pers.) in terms of above-ground productivity and metal accumulation, and to establish the role of root colonisation. Greater root lengths were generally correlated with metal concentrations in plant tissues but not with mass removals. The excessively high metal concentrations, both total and DTPA-extractable, of the wastes in ploughing cause worse rooting than subsoiling in almost all species. In both woody and crop species, the mass balance of phytoextraction mainly involved Zn and was relatively poor because of generalised impaired growth. However, poplar and willow were able to stabilise a considerable fraction of trace elements at below ground, making the application of phytostabilisation (coarse roots) through short-rotation coppicing a reliable complementary strategy. Instead, this possibility seems less practicable in herbaceous crops through their finer roots, although more precise quantification of root production and turnover is required for insights into long-term metal stabilisation in tap-rooted species. Results obtained at the Woolston New Cut Canal in the UK, which are an integral part of this thesis, indicate that stabilisation of several metals can also be achieved by adding soil with a small percentage of cement (1%) or lime (1%). This would reduce risks of metal movements through windblow and leachate transfer, and allow plants to improve their above-ground productivity. Among crops, Raphanus sativus var. oleiformis showed the highest tolerance to pyrite wastes, providing interesting phytoextraction balances, thanks to less altered growth rather than increased metal concentrations. The consistency of these results with past experiments in another polluted site suggested that this species can be conveniently exploited by testing various methods of improvement in metal uptake (e.g., auxins, humic acids, chelators). In pot trials, repeated soil applications of indolebutyric acid (IBA) have shown detrimental effects on plant growth and phytoextraction, regardless of doses of 0.1 and 1 mg kg-1. These negative effects were not observed for foliar spraying at 10 IBA mg L-1. Conversely, significant increases in root length and metal removal were seen after soil application of humic acids (HA) at small doses (0.1 g HA kg-1 of substrate), probably because of their hormone-like and chelating (weak) effects. Higher doses of HA (1 g kg-1), an extremely expensive option on a large scale, were fortunately not effective, as worsening of shoot biomass greatly exceeded improvements in metal concentrations. The application of ethylene-diamine-disuccinic acid (EDDS), a less persistent and more easily degraded chelator than the traditional ethylene-diamine-tetraacetic acid (EDTA), was experimented in both Raphanus sativus var. oleiformis and Brassica carinata A. Braun, but only occasionally were good phytoextraction results observed. EDDS always improved metal concentrations, regardless of doses of 2.5 and 5 mmol kg-1 (applications close to harvest), but biomass reduction was a prevailing effect. Repeated low-dosage applications of EDDS (1 mg kg-1 × 5 times) during the cycle, another possible strategy of chelator management, was less effective because of considerable phytotoxitoxicity and undesirable marked increases in metal leaching. Like those found for other chelators, these results suggest that, for maximum effectiveness and absence of environmental risks, EDDS should be applied one week before harvest at a moderate dose (2.5 mmol kg-1), regardless of species. It is concluded that phytomanagement of metal-polluted sites with field crops and coppices undoubtedly has a landscape value, but may only seldom become a reliable phytoextraction strategy in extreme contamination conditions like those due to pyrite wastes. In these circumstances, phytostabilisation is an option which should be evaluated more carefully as additional complementary mean. For both phytoextraction and stabilisation, the proper choice of species and adjustment of its cultivation method, as well as substantial soil amelioration and identification of specific treatments, can significantly improve these green technologies.Le attività civili, industriali e agricole sono responsabili del rilascio di inquinanti nell’ambiente. Tra le diverse fonti, la contaminazione del suolo da parte di metalli e matalloidi è una problematica di grande attualità. Contestualmente, la legislazione in materia di gestione e protezione ambientale di molti Paesi attribuisce alle tecnologie di bonifica biologiche, meno costose e di minor impatto ambientale, una rinnovata importanza, al pari dei più invasivi metodi di depurazione fisico-chimici. Il presente studio riguarda il primo tentativo in Italia di applicazione della fitodecontaminazione in situ di un’area – sita a Torviscosa, Udine – marcatamente inquinata da metalli e inclusa nell’elenco nazionale dei siti prioritari per la bonifica (D.M. 468/2001). A partire dagli anni Trenta e per diversi anni, nel sito sono state scaricate ceneri di pirite derivanti dall’arrostimento del minerale pirite per l’estrazione dello zolfo. Le ceneri sono risultate marcatamente contaminate da vari metalli come arsenico, cadmio, rame, piombo e zinco. A termine del periodo industriale, lo scarto industriale è stato ricoperto con uno strato di 0,15 m di terreno di riporto non inquinato ma ricco in scheletro. A causa delle particolari caratteristiche negative del substrato, struttura fisica alterata, carenza di nutrienti e delle sfavorevoli condizioni idrologiche, la flora spontanea che ha colonizzato negli anni il sito è risultata essere rada e inadeguata allo svolgimento di un efficiente processo fitoestrattivo. La realizzazione dell’impianto di fitoestrazione con specie da biomassa ha richiesto una cura particolare nella gestione del suolo e un adeguamento delle tecniche di coltivazione. In merito alla lavorazione del terreno, la coltivazione delle piante è avvenuta dopo aratura ed in comparazione con tecniche di ripuntatura, rispettivamente al fine di diluire le ceneri con il terreno di riporto e di mantenere uno strato superficiale non inquinato. In situ sono state coltivate specie arboree (Populus alba L., P. nigra L., P. tremula L., Salix alba L.) ed erbacee di interesse agrario (Helianthus annuus L., Lolium multiflorum Lam., Medicago sativa L., Raphanus sativus L. var. oleiformis Pers.), valutando la produzione di biomassa, l’accumulo di metalli ed il possibile ruolo dell’apparato radicale. In generale, un maggiore accrescimento radicale (lunghezza) è risultato correlato positivamente alla concentrazione di metalli raggiunta nei tessuti epigei, ma non con la loro rimozione. Nelle ceneri, l’aratura ha peggiorato l’accrescimento radicale di quasi tutte le specie saggiate a causa della maggiore presenza (totale e biodisponibile) di metalli rispetto alla ripuntatura. Nel sito di Torviscosa, l’asportazione di inquinanti è risultata generalmente modesta a causa della scarsa produttività vegetale, e ha riguardato quasi esclusivamente lo zinco. Pioppo e salice sono stati comunque in grado di stabilizzare una porzione considerevole di metalli pesanti a livello radicale, suggerendo che i sistemi SRC (Short Rotation Coppice) possano fornire anche un importante contributo nella fitostabilizzazione. Questa possibilità sembra invece essere meno realistica per le radici fibrose delle colture erbacee, a causa del loro intenso e rapido turnover. Si rende tuttavia necessaria una precisa quantificazione della produttività radicale e del tempo di ritenzione dei metalli nelle radici fittonanti. I risultati ottenuti presso il Woolston New Cut Canal (Inghilterra, UK), parte integrante di questa tesi, indicano che l’ammendamento della matrice inquinata con piccole quantità di cemento (1%) o di carbonato di calcio (1%) può contribuire a stabilizzare diversi metalli pesanti. Questo ridurrebbe la dispersione atmosferica e il dilavamento degli inquinanti, permettendo allo stesso tempo alle piante di migliorare la produttività per effetto della minore frazione di metalli biodisponibili. Tra le colture saggiate, il rafano (Raphanus sativus var. oleiformis) ha dimostrato un elevato grado di adattamento alle ceneri di pirite, raggiungendo un buon potenziale produttivo e interessanti asportazioni di metalli. L’interesse per questa specie è stato pertanto esteso anche a sperimentazioni in ambiente controllato per studiare l’applicazione di auxine, acidi umici e sostanze chelanti. L’applicazione ripetuta al suolo di acido indolbutirrico (IBA) non ha fornito i risultati attesi avendo ridotto l’accrescimento vegetale e l’accumulo di metalli sia a 0,1 che 1 mg IBA kg-1. L’ormone ha invece fatto incrementare la lunghezza radicale tramite applicazioni fogliari a 10 mg IBA L-1. Stimolazione dell’accrescimento radicale e miglioramenti del bilancio fitoestrattivo sono stati ottenuti con modeste applicazioni al suolo (0,1 g kg-1) di acidi umici (HA), verosimilmente per l’attività ormono-simile e del debole effetto chelante. Dosi superiori di HA (1 g kg-1), un’opzione estremamente costosa su larga scala, non sono risultate efficaci, a causa del prevalente effetto fitotossico. L’applicazione di acido etilendiaminodisuccinico (EDDS), un chelante meno persistente e più facilmente biodegradabile del ben noto acido etilendiaminotetracetico (EDTA), è stata sperimentata sia in Raphanus sativus var. oleiformis che in Brassica carinata A. Braun, ma solo raramente si sono osservati miglioramenti della fitoestrazione. L’EDDS ha infatti sempre aumentato la concentrazione di metalli nei tessuti epigei, indipendentemente dalla dose (2,5 e 5 mmol kg-1) applicata poco prima della raccolta, ma la riduzione di biomassa è stata più marcata. Un’altra strategia possibile di gestione del chelante, ovvero applicazioni ripetute a basso dosaggio durante il ciclo colturale (1 mmol kg-1 × 5 volte), è risultata meno efficace a causa della considerevole fitotossicità e dell’aumento incontrollato di metalli lisciviati. Questi risultati suggeriscono che l’EDDS, analogamente ad altri chelanti, possa essere applicato proficuamente e senza rischi ambientali una settimana prima della raccolta a dosi moderate (2,5 mmol kg-1), indipendentemente dalla specie vegetale considerata. Si può concludere che la gestione di siti inquinati da metalli pesanti tramite coperture vegetali agrarie e forestali possiede indubbiamente un importante valore paesaggistico, ma solo raramente può diventare una strategia concreta in condizioni estreme come quelle causate dalle ceneri di pirite. In queste circostanze la fitostabilizzazione è un’opzione da valutare con attenzione ed in termini di complementarietà alla fitoestrazione. Una corretta scelta delle specie e il perfezionamento delle tecniche agronomiche, così come il miglioramento delle condizioni chimico-fisiche della matrice inquinata e l’individuazione di trattamenti specifici, possono tuttavia rendere queste tecnologie ecocompatibili più efficaci e applicabili su larga scala

A Multi-disciplinary Challenge for Phytoremediation of Metal-Polluted Pyrite Waste

After the discovery of hyperaccumulators plants which accumulate high above-ground levels of one or a few metals without evident symptoms of toxicity, the application of plant-based technologies to remedy metal-contaminated soils has received huge attention. Metalliferous soils provide several hyperaccumulators, but their application must be verified carefully in terms of biomass\u2014generally very small\u2014and uptake when plants are cultivated out of their native environment. Phytoextraction consists of removing toxic elements through the harvestable biomass, after sufficient translocation from roots has occurred. Although promising, the method has some limitations due to difficult plant establishment, possible limited soil metal availability, insufficient root uptake (exclusion), symplastic mobility and xylem loading, as well as the great energy costs required for detoxification and storage